Computed tomography (CT) technology has been in use in the medical field for several years. Such CT scanning instruments produce a cross-sectional view through the subject material along any chosen axis. A two dimensional X-ray image of electron density variations within the object scanned is produced. The advantages of CT scanning over conventional radiography is found in its much clearer images and its superior resolution of variation in density of the object imaged.
In medical CT scanners, an X-ray source and a detector array circle a patient in a period of about 2 to 9 seconds and produce an image with maximum resolution of 0.25 mm in the X-Y plane. This plane can be moved in discrete intervals to obtain information in 3 dimensions. For more details of such medical CT scanners, reference may be made to U.S. Pat. No. 4,157,472 to Beck, Jr. and Barrett (assignee General Electric Company) and U.S. Pat. No. 4,399,509 to Hounsfield (assignee EMI Limited).
Many other applications of CT scanning can also be made. For example, CT scanning has been applied to the non-destructive testing of wood materials, such as for disease in living trees; see U.S. Pat. No. 4,283,629 to Habermehl. In a further application, CT scanning has been applied to the examination of non-living objects, such as motors, ingots, pipes, etc.; see U.S. Pat. No. 4,422,177 to Mastronardi et al (assignee American Science and Engineering, Inc.).
More recently, CT scanning technology has been applied to the field of energy research by way of petrophysical and reservoir engineering applications; see Wellington et al, "X-ray Computerized Tomography", Journal of Petroleum Technology, pages 885-898, August, 1987. Wellington et al describe generation of images of core samples from particular formations of interest. Wellington et al effectively acknowledge in FIG. 6 and discussion thereof, however, that the bulk density map of a core sample can be calculated accurately only if one already knows the lithology of the core material, that is, one must know the composition of the core in advance. This is a very substantial limitation on the use of CT scanning in analysis of rock samples.
More particularly, Wellington et al describes generation of images of core samples by CT scanning. However, Wellington et al acknowledges that to do so requires that one already know the lithology of the sample. This is often not possible. This is because it is impossible in many cases to examine core samples without destroying them. As noted by Wellington et al at page 889, such samples typically amount to unconsolidated samples of sand, frozen within opaque plastic tubes or the like. If the sample is removed from the tube, it typically collapses, destroying any structural integrity it may have had. This prevents determination of the bulk density .rho., and destroys highly relevant permeability and porosity information as well. No acceptable method for removing a "plug" from the core without compromising its structural integrity is known. Even if a particular sample itself is relatively consolidated, infiltration by drilling mud, for example, which commonly varies with depth, will interfere with density measurement. Thus, in general, accurate measurement of the density of a given undisturbed core sample is of primary interest in analysis thereof with respect to the search for oil and gas or the production of oil and gas from known reservoirs thereof.
Processes involved in coal gasification and combustion have been monitored using time-lapse CT imagery to observe changes in density (e.g., due to thermal expansion, fracturing, emission of gases, consumption of combustion) during progressive heating in a controlled atmosphere. Core flooding experiments can now be carried out with CT scanning to aid in enhanced oil recovery and fluid mobility control. For example, the permeability of materials within core samples to various fluids at varying conditions of temperature and pressure can be determined. Such experiments might involve flushing a fluid through a core sample and monitoring the shape of the fluid front. By subtracting the images of the cores before and after flooding, the exact shapes of the fluid front can be determined. Such core flood experiments allow the interior of the core sample to be observed without disturbing the sample. The sweep efficiency and flow paths of fluid of interest may then be studied on the scale of millimeters. Typically, the penetration of X-rays allows experiments to be performed with up to 4 inch diameter core samples. Relatively porous samples of somewhat greater dimension can also be analyzed.
Drilling fluids can usefully be analyzed by CT scanning, as such fluids are characterized by high density brines, various organics and several compositionally different weighing agents. Formation damage can also be investigated since CT scanning can detect infiltration of drilling mud, absorption of organics and the reversibility of completion fluid penetration. Shale oil recovery can be aided as CT scanning could detect penetration by solvents and could directly measure structure changes on retorting. Rock fractures can be identified.
U.S. Pat. No. 4,649,483 to Dixon discloses a method for determining fluid saturation in a porous medium through the use of CT scanning. Multi-phase fluid saturation in a sample of a porous medium is determined through computer tomographic scanning. The sample is scanned with X-rays of differing energies in both the fluid saturated and the fluid extracted states. Each of the extracted fluids is also scanned at differing X-ray energies. The computed tomographic images produced are utilized in the determination of the X-ray mass attenuation coefficients for the sample and the extracted fluids. From these mass attenuation coefficients, the weight fractions and volume fractions of each of the extracted fluids are determined.
U.S. Pat. No. 4,688,238 to Sprunt et al discloses a method for using CT scanning over a range of confining pressures on a core sample to determine pore volume change, pore compressibility and core fracturing. A core sample with a surrounding elastic jacket is placed in a confining pressure cell. Pressure is applied to the cell to press the jacket into contact with the surface of the sample. The pressure in the cell is increased stepwise over a plurality of pressure points. The sample is scanned at a plurality of locations with X-rays at each of the pressure points. Computed tomographic images of the sample are produced for each of the X-ray scans. The conformance of the jacket to the sample is determined from these computed tomographic images. From such conformance, a range of confining pressures is determined over which pore volume and pore compressibility of the sample are measured without being affected by improper conformance of the jacket to the surface of the sample. Also rock fracturing is determined from the pressure at which crushing of the sample destroys permeable channels within the sample and results in impermeability of the sample.
U.S. Pat. No. 4,722,095 to Muegge et al discloses a method for identifying porosity and drilling mud invasion of a core sample from a subterranean formation. A gas is supplied to the core sample at a first pressure. The gas is thereafter allowed to expand from the core sample until equilibrium is reached. The volume of the gas which expands from the core sample is measured. A second pressure is measured in the core sample after the gas has expanded. The pore volume of the core sample is determined from such first and second pressures and such measured gas volume. Bulk volume of the core sample is measured. Porosity of the core sample is determined from the ratio of the pore volume to the bulk volume. The core sample is scanned with X-rays and computer tomographic images produced. The concentration of drilling mud solids in the core sample is identified from the density effect of the drilling mud solid on the computer tomographic images. The porosity determination is corrected for the volumetric concentration of drilling mud solids in the pore spaces of the core sample. The X-ray scanning is calibrated to a density scale based on the barite content of the drilling mud, the barite having a higher grain density than the remaining minerals forming the core sample. This X-ray scanning is preferably calibrated by adjusting the computed tomographic number scale in Hounsfield units to a zero level based on the numerical measure of the X-ray absorption property of barite.
U.S. Pat. No. 4,799,382 to Sprunt et al discloses a method for measuring reservoir characteristics of a core sample from a subsurface formation by subjecting the core sample to pressure cycling. Pore volume changes during such pressure cycling are measured. Pore compressibility is determined from a plot of the measured pore volume change versus pressure. The core sample is scanned with X-rays at least once for each pressure cycle and a computed tomographic image is produced. From the plurality of produced images, a determination is made of the pressure at which fracturing initiated and of the location or locations within the sample at which fracturing occurs.
U.S. Pat. No. 4,782,501 to Dixon discloses a method for determining the porosity and pore compressibility of a core sample of a porous media by the use of X-ray scanning in the presence of confining pressure. The porosity of the core sample is determined at atmospheric reference pressure (i.e., zero confining stress). The sample is then saturated with a fluid of predetermined X-ray attenuation coefficient. The fluid saturated sample is placed in a confining pressure cell and scanned with X-rays at a plurality of points along the sample. A first set of computed tomographic images are produced at the plurality of points along the sample. A first set of computed tomographic images are produced at the plurality of points along the sample. From these images, an X-ray attenuation coefficient at zero confining stress is determined. The pressure is then increased within the cell to provide confining stress to the sample. The sample is then scanned with X-rays at a plurality of points along the sample. A second set of computed tomographic images are produced at said plurality of points along the sample for the confining stress. From these images, an X-ray attenuation at confining stress is determined. Porosity is then determined at confining stress from the determinations of sample porosity at zero confining stress, saturating fluid X-ray attenuation coefficient, sample X-ray attenuation coefficient at zero confining stress, and sample X-ray attenuation coefficient at confining stress.